Democratic Rep. Susan A. Davis, who has represented her Southern California district since 2001, announced Wednesday that she would not seek reelection in 2020.

Davis, 75, is the fourth Democratic House member to decide to resign or run for a different office, joining 15 Republicans who have decided to leave Congress.

In a letter to her constituents, Davis did not say why she was leaving but expressed a desire to live and work in San Diego after 20 years of commuting cross-country. She wrote that she had “struggled to make this very difficult decision.”

“This is tough for me. There are always compelling issues and challenges that have encouraged me to pursue each new election cycle,” Davis wrote. “Most touching has been the appeals to stay on and go yet another round with the hope that I would always be your representative. It doesn’t get any better than that!”

Davis’s district is solidly Democratic — Hillary Clinton won it over President Trump in 2016 by 35 points — so there is little risk to her party that her leaving would result in a Republican pickup.

Other Democrats who have announced their retirements are Reps. José E. Serrano (N.Y.), David Loebsack (Iowa) and Ben Ray Luján (N.M.), who is running for the Senate.